It is necessary for a future optical network to perform, for example, processing, such as branching and inserting of a signal light and switching of the same, at an apparatus, such as a repeating optical node, which is placed at a location remote from a terminal apparatus, while using a conventional optical communication system as a basis. In doing the above, it is effective from a viewpoint of energy efficiency to propagate and process information such that as little conversion as possible is performed between an optical signal and an electric signal.
However, an existing repeating optical node or the like performs signal processing by photoelectric conversion as performed in a terminal apparatus, and for example, a transmitted signal light is once converted to an electric signal, and this electric signal is electrically processed, whereafter the processed electric signal is converted to an optical signal again. This complicates the apparatus configuration, and further necessitates large electric power to perform photoelectric conversion.
By the way, in an optical network, various kinds of information are monitored at various points on a real-time basis, and effective network control is performed based on the monitored information. In a future optical network, the amount of such information increases, and hence it becomes effective to realize an energy-saving optical network. Further, to realize a more flexible optical network, a function is effective in which information is inserted into the network not only at an optical node, but also at a desired point.
However, at present, insertion of information is performed at an optical node apparatus or a terminal apparatus, and particularly, monitored information is propagated e.g. by performing photoelectric conversion of a signal light and writing the information into a header part of the signal light, or by using a dedicated optical wave.
Note that a transmission technique is known in which in a relay station, which is disposed between a transmitting station and a receiving station via an optical transmission path, there are provided a phase conjugate light-generating device that has signal light/pump light-supplying means for supplying a signal light input from the transmitting station and a pump light to a nonlinear optical medium, and signal light/phase conjugate light-extracting means for extracting an output signal light generated by modulating the input signal light by the pump light, and a phase conjugate light, using the input signal light and the pump light supplied to the nonlinear optical medium, and modulation means for modulating the pump light based on monitored data specific to the relay station, wherein the signal light containing the modulated monitored data and the phase conjugate light are transmitted to the receiving apparatus station (e.g. see Japanese Patent No. 3436310).
As described above, the conventional optical network has a problem that due to photoelectric conversion performed on a signal light, a large power loss is caused by insertion of another signal or propagation of information.